"What If" mode would allow us to set a checkpoint where we're going on a hunch.

Works like this:
Check the "What If" box.
The game then makes the green dot and red X both a shade of gray.
"Commit" and "Undo" buttons appear.
I can click around the grid on any white space with my gray marks.
"Commit" turns off "What If" mode and makes the gray marks the appropriate colors.
"Undo" clears all gray marks getting me back to where I entered "What If" mode.